Internet Engineering Task Force IPTEL WG
Internet Draft Lennox/Wu/Schulzrinne
Columbia University
draft-ietf-iptel-cpl-09.txt
April XX, 2004
Expires: October, 2004
CPL: A Language for User Control of Internet Telephony Services
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
The Call Processing Language (CPL) is a language that can be used to
describe and control Internet telephony services. It is designed to
be implementable on either network servers or user agent servers. It
is meant to be simple, extensible, easily edited by graphical
clients, and independent of operating system or signalling protocol.
It is suitable for running on a server where users may not be allowed
to execute arbitrary programs, as it has no variables, loops, or
ability to run external programs.
This document is a product of the IP Telephony (IPTEL) working group
of the Internet Engineering Task Force. Comments are solicited and
should be addressed to the working group's mailing list at
iptel@ietf.org and/or the authors.
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Internet Draft CPL April XX, 20041 Introduction
The Call Processing Language (CPL) is a language that can be used to
describe and control Internet telephony services. It is not tied to
any particular signalling architecture or protocol; it is anticipated
that it will be used with both the Session Initiation Protocol (SIP)
[1] and H.323 [16].
CPL is powerful enough to describe a large number of services and
features, but it is limited in power so that it can run safely in
Internet telephony servers. The intention is to make it impossible
for users to do anything more complex (and dangerous) than describing
Internet telephony services. The language is not Turing-complete, and
provides no way to write loops or recursion.
CPL is also designed to be easily created and edited by graphical
tools. It is based on the Extensible Markup Language (XML) [2], so
parsing it is easy and many parsers for it are publicly available.
The structure of the language maps closely to its behavior, so an
editor can understand any valid script, even ones written by hand.
The language is also designed so that a server can easily confirm
scripts' validity at the time they are delivered to it, rather that
discovering them while a call is being processed.
Implementations of CPL are expected to take place both in Internet
telephony servers and in advanced clients; both can usefully process
and direct users' calls. This document primarily addresses the usage
in servers. A mechanism will be needed to transport scripts between
clients and servers; this document does not describe such a
mechanism, but related documents will.
The framework and requirements for the CPL architecture are described
in RFC 2824, "Call Processing Language Framework and Requirements"
[17].
1.1 Conventions of This Document
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [3] and
indicate requirement levels for compliant CPL implementations.
Some paragraphs are indented, like this; they give
motivations of design choices, advice to implementors, or
thoughts on future development of or extensions to CPL.
They are not essential to the specification of the
language, and are non-normative.
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Internet Draft CPL April XX, 20042 Structure of CPL Scripts2.1 High-level Structure
A CPL script consists of two types of information: ancillary
information about the script, and call processing actions.
A call processing action is a structured tree that describes the
operations and decisions a telephony signalling server performs on a
call set-up event. There are two types of call processing actions:
top-level actions and subactions. Top-level actions are actions that
are triggered by signalling events that arrive at the server. Two
top-level actions are defined: "incoming", the action performed when
a call arrives whose destination is the owner of the script; and
"outgoing", the action performed when a call arrives whose originator
is the owner of the script. Subactions are actions which can be
called from other actions. CPL forbids subactions from being called
recursively: see Section 8.
Ancillary information is information which is necessary for a server
to correctly process a script, but which does not directly describe
any operations or decisions. Currently, no ancillary information is
defined, but the section is reserved for use by extensions.
2.2 Abstract Structure of a Call Processing Action
Abstractly, a call processing action is described by a collection of
nodes, which describe operations that can be performed or decisions
that can be made. A node may have several parameters, which specify
the precise behavior of the node; they usually also have outputs,
which depend on the result of the decision or action.
For a graphical representation of a CPL action, see Figure 1. Nodes
and outputs can be thought of informally as boxes and arrows; CPL is
designed so that actions can be conveniently edited graphically using
this representation. Nodes are arranged in a tree, starting at a
single root node; outputs of nodes are connected to additional nodes.
When an action is run, the action or decision described by the
action's top-level node is performed; based on the result of that
node, the server follows one of the node's outputs, and the
subsequent node it points to is performed; this process continues
until a node with no specified outputs is reached. Because the graph
is acyclic, this will occur after a bounded and predictable number of
nodes are visited.
If an output to a node does not point to another node, it indicates
that the CPL server should perform a node- or protocol-specific
action. Some nodes have specific default behavior associated with
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them; for others, the default behavior is implicit in the underlying
signalling protocol, or can be configured by the administrator of the
server. For further details on this, see Section 10.
_________________ ___________________ ________ busy
| Address-switch | | location | | proxy |--------\
Call-->| field: origin | ->| url: sip:jones@ |->|timeout:| timeout|
| subfield: host | / | example.com | | 10s |--------|
|-----------------|/ |___________________| | | failure|
| subdomain-of: | |________|--------|
| example.com | |
|-----------------| ___________________________________________/
| otherwise | /........................................
| |\|. Voicemail .
|_________________| \. ____________________ .
->| location | __________ .
. | url: sip:jones@ | | redirect | .
. | voicemail. |->| | .
. | example.com | |__________| .
. |____________________| .
........................................
Figure 1: Sample CPL Action: Graphical Version
2.3 Location Model
For flexibility, one piece of information necessary for CPL is not
given as node parameters: the set of locations to which a call is to
be directed. Instead, this set of locations is stored as an implicit
global variable throughout the execution of a processing action (and
its subactions). This allows locations to be retrieved from external
sources, filtered, and so forth, without requiring general language
support for such operations (which could harm the simplicity and
tractability of understanding the language). The specific operations
which add, retrieve, or filter location sets are given in Section 5.
For the incoming top-level call processing action, the location set
is initialized to the empty set. For the outgoing action, it is
initialized to the destination address of the call.
2.4 XML Structure
Syntactically, CPL scripts are represented by XML documents. XML is
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thoroughly specified by the XML specification [2], and implementors
of this specification should be familiar with that document, but as a
brief overview, XML consists of a hierarchical structure of tags;
each tag can have a number of attributes. It is visually and
structurally very similar to HTML [18], as both languages are
simplifications of the earlier and larger standard SGML [19].
See Figure 2 for the XML document corresponding to the graphical
representation of the CPL script in Figure 1. Both nodes and outputs
in CPL are represented by XML tags; parameters are represented by XML
tag attributes. Typically, node tags contain output tags, and vice-
versa (with a few exceptions: see Sections 5.1, 5.3, 7.1, and 7.2).
The connection between the output of a node and another node is
represented by enclosing the tag representing the pointed-to node
inside the tag for the outer node's output. Convergence (several
outputs pointing to a single node) is represented by subactions,
discussed further in Section 8.
The higher-level structure of a CPL script is represented by tags
corresponding to each piece of ancillary information, subactions, and
top-level actions, in order. This higher-level information is all
enclosed in a special tag "cpl", the outermost tag of the XML
document.
A complete XML Schema for CPL is provided in Appendix C. The
remainder of the main sections of this document describe the
semantics of CPL, while giving its syntax informally. For the formal
syntax, please see the appendix.
3 Script Structure: Overview
As mentioned, a CPL script consists of ancillary information,
subactions, and top-level actions. The full syntax of the "cpl" node
is given in Figure 3.
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consist of a tree of nodes and outputs. Nodes and outputs are both
described by XML tags. There are four categories of CPL nodes:
switches, which represent choices a CPL script can make; location
modifiers, which add or remove locations from the location set;
signalling operations, which cause signalling events in the
underlying protocol; and non-signalling operations, which trigger
behavior which does not effect the underlying protocol.
4 Switches
Switches represent choices a CPL script can make, based on either
attributes of the original call request or items independent of the
call.
All switches are arranged as a list of conditions that can match a
variable. Each condition corresponds to a node output; the output
points to the next node to execute if the condition was true. The
conditions are tried in the order they are presented in the script;
the output corresponding to the first node to match is taken.
There are two special switch outputs that apply to every switch type.
The output "not-present", which MAY occur anywhere in the list of
outputs, is true if the variable the switch was to match was not
present in the original call setup request. (In this document, this
is sometimes described by saying that the information is "absent".)
The output "otherwise", which MUST be the last output specified if it
is present, matches if no other condition matched.
If no condition matches and no "otherwise" output was present in the
script, the default script behavior is taken. See Section 10 for more
information on this.
Switches MAY contain no outputs. They MAY contain only an "otherwise"
output.
Such switches are not particularly useful, but might be
created by tools which automatically generate CPL scripts.
4.1 Address Switches
Address switches allow a CPL script to make decisions based on one of
the addresses present in the original call request. They are
summarized in Figure 4.
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Node: "address-switch"
Outputs: "address" Specific addresses to match
Parameters: "field" "origin", "destination",
or "original-destination"
"subfield" "address-type", "user", "host",
"port", "tel", or "display"
(also: "password" and "alias-type")
Output: "address"
Parameters: "is" Exact match
"contains" Substring match (for "display" only)
"subdomain-of" Sub-domain match (for "host", "tel")
Figure 4: Syntax of the "address-switch" node
Address switches have two node parameters: "field", and "subfield".
The mandatory "field" parameter allows the script to specify which
address is to be considered for the switch: either the call's origin
address (field "origin"), its current destination address (field
"destination"), or its original destination (field "original-
destination"), the destination the call had before any earlier
forwarding was invoked. Servers MAY define additional field values.
The optional "subfield" specifies what part of the address is to be
considered. The possible subfield values are: "address-type", "user",
"host", "port", "tel", and "display". Additional subfield values MAY
be defined for protocol-specific values. (The subfield "password" is
defined for SIP in Section 4.1.1; the subfield "alias-type" is
defined for H.323 in Appendix B.1.) If no subfield is specified, the
"entire" address is matched; the precise meaning of this is defined
for each underlying signalling protocol. Servers MAY define
additional subfield values.
The subfields are defined as follows:
address-type This indicates the type of the underlying address;
i.e., the URI scheme, if the address can be represented by
a URI. The types specifically discussed by this document
are "sip", "tel", and "h323". The address type is not
case-sensitive. It has a value for all defined address
types.
user This subfield of the address indicates, for e-mail style
addresses, the user part of the address. For telephone
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number style address, it includes the subscriber number.
This subfield is case-sensitive; it may be absent.
host This subfield of the address indicates the Internet host
name or IP address corresponding to the address, in host
name, IPv4, or IPv6 [4] textual representation format.
Host names are compared as strings. IP addresses are
compared numerically. (In particular, the presence or
location of an IPv6 :: omitted-zero-bits block is not
significant for matching purposes.) Host names are never
equal to IP addresses -- no DNS resolution is performed.
IPv4 addresses are never equal to IPv6 addresses, even if
the IPv6 address is a v4-in-v6 embedding. This subfield is
not case sensitive, and may be absent.
For host names only, subdomain matching is supported with
the "subdomain-of" match operator. The "subdomain-of"
operator ignores leading dots in the hostname or match
pattern, if any.
port This subfield indicates the TCP or UDP port number of the
address, numerically in decimal format. It is not case
sensitive, as it MUST only contain decimal digits. Leading
zeros are ignored.
tel This subfield indicates a telephone subscriber number, if
the address contains such a number. It is not case
sensitive (the telephone numbers may contain the symbols
`A' `B' `C' and `D'), and may be absent. It may be matched
using the "subdomain-of" match operator. Punctuation and
separator characters in telephone numbers are discarded.
display This subfield indicates a "display name" or user-visible
name corresponding to an address. It is a Unicode string,
and is matched using the case-insensitive algorithm
described in Section 4.2. The "contains" operator may be
applied to it. It may be absent.
For any completely unknown subfield, the server MAY reject the script
at the time it is submitted with an indication of the problem; if a
script with an unknown subfield is executed, the server MUST consider
the "not-present" output to be the valid one.
The "address" output tag may take exactly one of three possible
parameters, indicating the kind of matching allowed.
is An output with this match operator is followed if the
subfield being matched in the "address-switch" exactly
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matches the argument of the operator. It may be used for
any subfield, or for the entire address if no subfield was
specified.
subdomain-of This match operator applies only for the subfields
"host" and "tel". In the former case, it matches if the
hostname being matched is a subdomain of the domain given
in the argument of the match operator; thus, subdomain-
of="example.com" would match the hostnames "example.com",
"research.example.com", and
"zaphod.sales.internal.example.com". IP addresses may be
given as arguments to this operator; however, they only
match exactly. In the case of the "tel" subfield, the
output matches if the telephone number being matched has a
prefix that matches the argument of the match operator;
subdomain-of="1212555" would match the telephone number "1
212 555 1212."
contains This match operator applies only for the subfield
"display". The output matches if the display name being
matched contains the argument of the match as a substring.
4.1.1 Usage of "address-switch" with SIP
For SIP, the "origin" address corresponds to the address in the
"From" header; "destination" corresponds to the "Request-URI"; and
"original-destination" corresponds to the "To" header.
The "display" subfield of an address is the display-name part of the
address, if it is present. Because of SIP's syntax, the "destination"
address field will never have a "display" subfield.
The "address-type" subfield of an address is the URI scheme of that
address. Other address fields depend on that "address-type".
For SIP URIs, the "user", "host", and "port" subfields correspond to
the "user," "host," and "port" elements of the URI syntax. (Note
that, following the definitions of RFC 3261 [1], a SIP URI which does
not specify a port is not the same as an explicit port 5060; the
former is indicated by an absent port subfield.) The "tel" subfield
is defined to be the "user" part of the URI, with visual separators
stripped, if the "user=phone" parameter is given to the URI, or if
the server is otherwise configured to recognize the user part as a
telephone number. An additional subfield, "password" is defined to
correspond to the "password" element of the SIP URI, and is case-
sensitive. However, use of this field is NOT RECOMMENDED for general
security reasons.
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For tel URLs, the "tel" and "user" subfields are the subscriber name;
in the former case, visual separators are stripped. The "host" and
"port" subfields are both not present.
For h323 URLs, subfields MAY be set according to the scheme described
in Appendix B.
For other URI schemes, only the "address-type" subfield is defined by
this specification; servers MAY set other pre-defined subfields, or
MAY support additional subfields.
If no subfield is specified for addresses in SIP messages, the string
matched is the URI part of the address. For "is" matches, standard
SIP URI matching rules are used; for "contains" matches, the URI is
used verbatim.
4.2 String Switches
String switches allow a CPL script to make decisions based on free-
form strings present in a call request. They are summarized in Figure
5.
Node: "string-switch"
Outputs: "string" Specific string to match
Parameters: "field" "subject", "organization",
"user-agent", or "display"
Output: "string"
Parameters: "is" Exact match
"contains" Substring match
Figure 5: Syntax of the "string-switch" node
String switches have one node parameter: "field". The mandatory
"field" parameter specifies which string is to be matched.
String switches are dependent on the call signalling protocol being
used.
Five fields are defined, listed below. The value of each of these
fields, except as specified, is a free-form Unicode string with no
other structure defined.
"subject" The subject of the call.
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"organization" The organization of the originator of the call.
"user-agent" The name of the program or device with which the
call request was made.
"display" Free-form text associated with the call, intended to
be displayed to the recipient, with no other semantics
defined by the signalling protocol.
Strings are matched as case-insensitive Unicode strings, in the
following manner. First, strings are canonicalized to the
"Compatibility Composition" (KC) form, as specified in Unicode
Technical Report 15 [5]. Then, strings are compared using locale-
insensitive caseless mapping, as specified in Unicode Technical
Report 21 [6].
Code to perform the first step, in Java and Perl, is
available; see the links from Annex E of UTR 15 [5]. The
case-insensitive string comparison in the Java standard
class libraries already performs the second step; other
Unicode-aware libraries should be similar.
The output tag of string matching is named "string", and has a
mandatory argument, one of "is" or "contains", indicating whole-
string match or substring match, respectively.
4.2.1 Usage of "string-switch" with SIP
For SIP, the fields "subject", "organization", and "user-agent"
correspond to the SIP header fields with the same name. These are
used verbatim as they appear in the message.
The field "display" is not used, and is never present.
4.3 Language Switches
Language switches allow a CPL script to make decisions based on the
languages in which the originator of the call wishes to communicate.
They are summarized in Figure 6.
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Node: "language-switch"
Outputs: "language" Specific string to match
Parameters: None
Output: "language"
Parameters: "matches" Match if the given language matches a
language-range of the call.
Figure 6: Syntax of the "language-switch" node
Language switches take no parameters.
The "language" output takes one parameter, "matches". The value of
the parameter is a language-tag, as defined in RFC 3066 [7]. The
caller may have specified a set of language-ranges, also as defined
in RFC 3066. The CPL server checks each language-tag specified by the
script against the language-ranges specified in the request.
See RFC 3066 for the details of how language-ranges match language-
tags. Briefly, a language-range matches a language-tag if it exactly
equals the tag, or if it exactly equals a prefix of the tag such that
the first character following the prefix is "-".
If the caller specified the special language-range "*", it is ignored
for the purpose of matching. Languages with a "q" value of 0 are also
ignored.
This switch MAY be not-present.
4.3.1 Usage of "language-switch" with SIP
The language-ranges for the "language-switch" switch are obtained
from the SIP "Accept-Language" header field. The switch is not-
present if the initial SIP request did not contain this header field.
Note that because of CPL's first-match semantics in
switches, "q" values other than 0 of the "Accept-Language"
header fields are ignored.
4.4 Time Switches
Time switches allow a CPL script to make decisions based on the time
and/or date the script is being executed. They are summarized in
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Figure 7.
Time switches are independent of the underlying signalling protocol.
Node: "time-switch"
Outputs: "time" Specific time to match
Parameters: "tzid" RFC 2445 Time Zone Identifier
"tzurl" RFC 2445 Time Zone URL
Output: "time"
Parameters: "dtstart" Start of interval (RFC 2445 DATE-TIME)
"dtend" End of interval (RFC 2445 DATE-TIME)
"duration" Length of interval (RFC 2445 DURATION)
"freq" Frequency of recurrence ("secondly",
"minutely", "hourly", "daily",
"weekly", "monthly", or "yearly")
"interval" How often the recurrence repeats
"until" Bound of recurrence (RFC 2445 DATE-TIME)
"count" Number of occurrences of recurrence
"bysecond" List of seconds within a minute
"byminute" List of minutes within an hour
"byhour" List of hours of the day
"byday" List of days of the week
"bymonthday" List of days of the month
"byyearday" List of days of the year
"byweekno" List of weeks of the year
"bymonth" List of months of the year
"wkst" First day of the work week
"bysetpos" List of values within
set of events specified
Figure 7: Syntax of the "time-switch" node
Time switches are based closely on the specification of recurring
intervals of time in the Internet Calendaring and Scheduling Core
Object Specification (iCalendar COS), RFC 2445 [8].
This allows CPL scripts to be generated automatically from
calendar books. It also allows us to re-use the extensive
existing work specifying time intervals.
If future standards-track documents are published that update or
obsolete RFC 2445, any changes or clarifications those documents make
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to recurrence handling apply to CPL time-switches as well.
An algorithm to determine whether an instant falls within a given
recurrence is given in Appendix A.
The "time-switch" tag takes two optional parameters, "tzid" and
"tzurl", both of which are defined in RFC 2445 (Sections 4.8.3.1 and
4.8.3.5 respectively). The "tzid" is the identifying label by which a
time zone definition is referenced. If it begins with a forward slash
(solidus), it references a to-be-defined global time zone registry;
otherwise it is locally-defined at the server. The "tzurl" gives a
network location from which an up-to-date VTIMEZONE definition for
the timezone can be retrieved.
While "tzid" labels that do not begin with a forward slash are
locally defined, it is RECOMMENDED that servers support at least the
naming scheme used by Olson Time Zone database [9]. Examples of
timezone databases that use the Olson scheme are the zoneinfo files
on most Unix-like systems, and the standard Java TimeZone class.
Servers SHOULD resolve "tzid" and "tzurl" references to time zone
definitions at the time the script is uploaded. They MAY periodically
refresh these resolutions to obtain the most up-to-date definition of
a time zone. If a "tzurl" becomes invalid, servers SHOULD remember
the most recent valid data retrieved from the URL.
If a script is uploaded with a "tzid" and "tzurl" which the CPL
server does not recognize or cannot resolve, it SHOULD diagnose and
reject this at script upload time. If neither "tzid" nor "tzurl" are
present, all non-UTC times within this time switch should be
interpreted as being "floating" times, i.e. that they are specified
in the local timezone of the CPL server.
Because of daylight-savings-time changes over the course of
a year, it is necessary to specify time switches in a given
timezone. UTC offsets are not sufficient, or a time-of-day
routing rule which held between 9 am and 5 pm in the
eastern United States would start holding between 8 am and
4 pm at the end of October.
Authors of CPL servers should be careful to handle correctly the
intervals when local time is discontinuous, at the beginning or end
of daylight-savings time. Note especially that some times may occur
more than once when clocks are set back. The algorithm in Appendix A
is believed to handle this correctly.
Time nodes specify a list of periods during which their output should
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be taken. They have two required parameters: "dtstart", which
specifies the beginning of the first period of the list, and exactly
one of "dtend" or "duration", which specify the ending time or the
duration of the period, respectively. The "dtstart" and "dtend"
parameters are formatted as iCalendar COS DATE-TIME values, as
specified in Section 4.3.5 of RFC 2445 [8]. Because time zones are
specified in the top-level "time-switch" tag, only forms 1 or 2
(floating or UTC times) can be used. The "duration" parameter is
given as an iCalendar COS DURATION parameter, as specified in section4.3.6 of RFC 2445. Both the DATE-TIME and the DURATION syntaxes are
subsets of the corresponding syntaxes from ISO 8601 [20].
For a recurring interval, the "duration" parameter MUST be small
enough such that subsequent intervals do not overlap. For non-
recurring intervals, durations of any positive length are permitted.
Zero-length and negative-length durations are not allowed.
If no other parameters are specified, a time node indicates only a
single period of time. More complicated sets periods intervals are
constructed as recurrences. A recurrence is specified by including
the "freq" parameter, which indicates the type of recurrence rule.
Parameters other than "dtstart", "dtend", and "duration" SHOULD NOT
be specified unless "freq" is present, though CPL servers SHOULD
accept scripts with such parameters present, and ignore the other
parameters.
The "freq" parameter takes one of the following values: "secondly",
to specify repeating periods based on an interval of a second or
more; "minutely", to specify repeating periods based on an interval
of a minute or more; "hourly", to specify repeating periods based on
an interval of an hour or more; "daily", to specify repeating periods
based on an interval of a day or more; "weekly", to specify repeating
periods based on an interval of a week or more; "monthly", to specify
repeating periods based on an interval of a month or more; and
"yearly", to specify repeating periods based on an interval of a year
or more. These values are not case-sensitive.
The "interval" parameter contains a positive integer representing how
often the recurrence rule repeats. The default value is "1", meaning
every second for a "secondly" rule, every minute for a "minutely"
rule, every hour for an "hourly" rule, every day for a "daily" rule,
every week for a "weekly" rule, every month for a "monthly" rule and
every year for a "yearly" rule.
The "until" parameter defines an iCalendar COS DATE or DATE-TIME
value which bounds the recurrence rule in an inclusive manner. If the
value specified by "until" is synchronized with the specified
recurrence, this date or date-time becomes the last instance of the
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recurrence. If specified as a date-time value, then it MUST be
specified in an UTC time format. If not present, and the "count"
parameter is not also present, the recurrence is considered to repeat
forever.
The "count" parameter defines the number of occurrences at which to
range-bound the recurrence. The "dtstart" parameter counts as the
first occurrence. The "until" and "count" parameters MUST NOT occur
in the same "time" output.
The "bysecond" parameter specifies a comma-separated list of seconds
within a minute. Valid values are 0 to 59. The "byminute" parameter
specifies a comma-separated list of minutes within an hour. Valid
values are 0 to 59. The "byhour" parameter specifies a comma-
separated list of hours of the day. Valid values are 0 to 23.
The "byday" parameter specifies a comma-separated list of days of the
week. "MO" indicates Monday; "TU" indicates Tuesday; "WE" indicates
Wednesday; "TH" indicates Thursday; "FR" indicates Friday; "SA"
indicates Saturday; "SU" indicates Sunday. These values are not
case-sensitive.
Each "byday" value can also be preceded by a positive (+n) or
negative (-n) integer. If present, this indicates the nth occurrence
of the specific day within the "monthly" or "yearly" recurrence. For
example, within a "monthly" rule, +1MO (or simply 1MO) represents the
first Monday within the month, whereas -1MO represents the last
Monday of the month. If an integer modifier is not present, it means
all days of this type within the specified frequency. For example,
within a "monthly" rule, MO represents all Mondays within the month.
The "bymonthday" parameter specifies a comma-separated list of days
of the month. Valid values are 1 to 31 or -31 to -1. For example, -10
represents the tenth to the last day of the month.
The "byyearday" parameter specifies a comma-separated list of days of
the year. Valid values are 1 to 366 or -366 to -1. For example, -1
represents the last day of the year (December 31st) and -306
represents the 306th to the last day of the year (March 1st).
The "byweekno" parameter specifies a comma-separated list of ordinals
specifying weeks of the year. Valid values are 1 to 53 or -53 to -1.
This corresponds to weeks according to week numbering as defined in
ISO 8601 [20]. A week is defined as a seven day period, starting on
the day of the week defined to be the week start (see "wkst"). Week
number one of the calendar year is the first week which contains at
least four (4) days in that calendar year. This parameter is only
valid for "yearly" rules. For example, 3 represents the third week of
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the year.
Note: Assuming a Monday week start, week 53 can only occur
when Thursday is January 1 or if it is a leap year and
Wednesday is January 1.
The "bymonth" parameter specifies a comma-separated list of months of
the year. Valid values are 1 to 12.
The "wkst" parameter specifies the day on which the work week starts.
Valid values are "MO", "TU", "WE", "TH", "FR", "SA" and "SU". This is
significant when a "weekly" recurrence has an interval greater than
1, and a "byday" parameter is specified. This is also significant in
a "yearly" recurrence when a "byweekno" parameter is specified. The
default value is "MO", following ISO 8601 [20].
The "bysetpos" parameter specifies a comma-separated list of values
which corresponds to the nth occurrence within the set of events
specified by the rule. Valid values are 1 to 366 or -366 to -1. It
MUST only be used in conjunction with another byxxx parameter. For
example "the last work day of the month" could be represented as:
<time -timerange- freq="monthly" byday="MO,TU,WE,TH,FR"
bysetpos="-1">
Each "bysetpos" value can include a positive (+n) or negative (-n)
integer. If present, this indicates the nth occurrence of the
specific occurrence within the set of events specified by the rule.
If byxxx parameter values are found which are beyond the available
scope (ie, bymonthday="30" in February), they are simply ignored.
Byxxx parameters modify the recurrence in some manner. Byxxx rule
parts for a period of time which is the same or greater than the
frequency generally reduce or limit the number of occurrences of the
recurrence generated. For example, freq="daily" bymonth="1" reduces
the number of recurrence instances from all days (if the "bymonth"
parameter is not present) to all days in January. Byxxx parameters
for a period of time less than the frequency generally increase or
expand the number of occurrences of the recurrence. For example,
freq="yearly" bymonth="1,2" increases the number of days within the
yearly recurrence set from 1 (if "bymonth" parameter is not present)
to 2.
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If multiple Byxxx parameters are specified, then after evaluating the
specified "freq" and "interval" parameters, the Byxxx parameters are
applied to the current set of evaluated occurrences in the following
order: "bymonth", "byweekno", "byyearday", "bymonthday", "byday",
"byhour", "byminute", "bysecond" and "bysetpos"; then "count" and
"until" are evaluated.
Here is an example of evaluating multiple Byxxx parameters.
<time dtstart="19970105T083000" duration="10M"
freq="yearly" interval="2" bymonth="1" byday="SU"
byhour="8,9" byminute="30">
First, the interval="2" would be applied to freq="yearly" to arrive
at "every other year." Then, bymonth="1" would be applied to arrive
at "every January, every other year." Then, byday="SU" would be
applied to arrive at "every Sunday in January, every other year."
Then, byhour="8,9" would be applied to arrive at "every Sunday in
January at 8 AM and 9 AM, every other year." Then, byminute="30"
would be applied to arrive at "every Sunday in January at 8:30 AM and
9:30 AM, every other year." Then the second is derived from "dtstart"
to end up in "every Sunday in January from 8:30:00 AM to 8:40:00 AM,
and from and 9:30:00 AM to 9:40:00 AM, every other year." Similarly,
if the "byminute", "byhour", "byday", "bymonthday" or "bymonth"
parameter were missing, the appropriate minute, hour, day or month
would have been retrieved from the "dtstart" parameter.
The iCalendar COS RDATE, EXRULE and EXDATE recurrence rules are not
specifically mapped to components of the time-switch node. Equivalent
functionality to the exception rules can be attained by using the
ordering of switch rules to exclude times using earlier rules;
equivalent functionality to the additional-date RDATE rules can be
attained by using "sub" nodes (see Section 8) to link multiple
outputs to the same subsequent node.
The "not-present" output is never true for a time switch. However, it
MAY be included, to allow switch processing to be more regular.
4.4.1 iCalendar differences and implementation issues
(This sub-sub-section is non-normative.)
The specification of recurring events in this section is identical
(except for syntax and formatting issues) to that of RFC 2445 [8],
with only one additional restriction. That one restriction is that
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consecutive instances of recurrence intervals may not overlap.
It was a matter of some debate, during the design of CPL, whether the
entire iCalendar COS recurrence specification should be included in
CPL, or whether only a subset should be included. It was eventually
decided that compatibility between the two protocols was of primary
importance. This imposes some additional implementation issues on
implementors of CPL servers.
It does not appear to be possible to determine, in constant time,
whether a given instant of time falls within one of the intervals
defined by a full iCalendar COS recurrence. The primary concerns are
as follows:
o The "count" parameter cannot be checked in constant running
time, since it requires that the server enumerate all
recurrences from "dtstart" to the present time, in order to
determine whether the current recurrence satisfies the
parameter. However, a server can expand a "count" parameter
once, off-line, to determine the date of the last recurrence.
This date can then be treated as a virtual "until" parameter
for the server's internal processing.
o Similarly, the "bysetpos" parameter requires that the server
enumerate all instances of the occurrence from the start of
the current recurrence set until the present time. This
requires somewhat more complex pre-processing, but generally,
a single recurrence with a "bysetpos" parameter can be split
up into several recurrences without them.
o Finally, constant running time of time switches also requires
that a candidate starting time for a recurrence can be
established quickly and uniquely, to check whether it
satisfies the other restrictions. This requires that a
recurrence's duration not be longer than its repetition
interval, so that a given instant cannot fall within several
consecutive potential repetitions of the recurrence. The
restriction that consecutive intervals not overlap partially
satisfies this condition, but does not fully ensure it. Again,
to some extent pre-processing can help resolve this.
The algorithm given in Appendix A runs in constant time after these
pre-processing steps.
Servers ought to check that recurrence rules do not create any absurd
run-time or memory requirements, and reject those that do, just as
they ought to check that CPL scripts in general are not absurdly
large.
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Priority switches allow a CPL script to make decisions based on the
priority specified for the original call. They are summarized in
Figure 8. They are dependent on the underlying signalling protocol.
Node: "priority-switch"
Outputs: "priority" Specific priority to match
Parameters: None
Output: "priority"
Parameters: "less" Match if priority is less
than that specified
"greater" Match if priority is greater
than that specified
"equal" Match if priority is equal
to that specified
Figure 8: Syntax of the "priority-switch" node
Priority switches take no parameters.
The "priority" tag takes one of the three parameters "greater",
"less", and "equal". The values of these parameters are one of the
following priorities: in decreasing order, "emergency", "urgent",
"normal", and "non-urgent". These values are matched in a case-
insensitive manner. Outputs with the "less" parameter are taken if
the priority of the call is less than the priority given in the
argument; and so forth.
If no priority is specified in a message, the priority is considered
to be "normal". If an unknown priority is specified in the call, it
is considered to be equivalent to "normal" for the purposes of
"greater" and "less" comparisons, but it is compared literally for
"equal" comparisons.
Since every message has a priority, the "not-present" output is never
true for a priority switch. However, it MAY be included, to allow
switch processing to be more regular.
4.5.1 Usage of "priority-switch" with SIP
The priority of a SIP message corresponds to the "Priority" header in
the initial "INVITE" message.
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Internet Draft CPL April XX, 20045 Location Modifiers
The abstract location model of CPL is described in Section 2.3. The
behavior of several of the signalling operations (defined in Section6) is dependent on the current location set specified. Location nodes
add or remove locations from the location set.
There are three types of location nodes defined. Explicit locations
add literally-specified locations to the current location set;
location lookups obtain locations from some outside source; and
location filters remove locations from the set, based on some
specified criteria.
5.1 Explicit Location
Explicit location nodes specify a location literally. Their syntax is
described in Figure 9.
Explicit location nodes are dependent on the underlying signalling
protocol.
Node: "location"
Outputs: None (Next node follows directly)
Next node: Any node
Parameters: "url" URL of address to add to location set
"priority" Priority of this location (0.0-1.0)
"clear" Whether to clear the location set before
adding the new value
Figure 9: Syntax of the "location" node
Explicit location nodes have three node parameters. The mandatory
"url" parameter's value is the URL of the address to add to the
location set. Only one address may be specified per location node;
multiple locations may be specified by cascading these nodes.
The optional "priority" parameter specifies a priority for the
location. Its value is a floating-point number between 0.0 and 1.0.
If it is not specified, the server SHOULD assume a default priority
of 1.0. The optional "clear" parameter specifies whether the location
set should be cleared before adding the new location to it. Its value
can be "yes" or "no", with "no" as the default.
Basic location nodes have only one possible result, since there is no
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way that they can fail. (If a basic location node specifies a
location which isn't supported by the underlying signalling protocol,
the script server SHOULD detect this and report it to the user at the
time the script is submitted.) Therefore, their XML representations
do not have explicit output tags; the <location> tag directly
contains another node.
5.1.1 Usage of "location" with SIP
All SIP locations are represented as URLs, so the locations specified
in "location" tags are interpreted directly.
5.2 Location Lookup
Locations can also be specified up through external means, through
the use of location lookups. The syntax of these tags is given in
Figure 10.
Location lookup is dependent on the underlying signalling protocol.
Node: "lookup"
Outputs: "success" Next node if lookup was successful
"notfound" Next node if lookup found no addresses
"failure" Next node if lookup failed
Parameters: "source" Source of the lookup
"timeout" Time to try before giving up on the lookup
"clear" Whether to clear the location set before
adding the new values
Output: "success"
Parameters: none
Output: "notfound"
Parameters: none
Output: "failure"
Parameters: none
Figure 10: Syntax of the "lookup" node
Location lookup nodes have one mandatory parameter and two optional
parameters. The mandatory parameter is "source", the source of the
lookup. This can either be a URI, or a non-URI value. If the value of
"source" is a URI, it indicates a location which the CPL server can
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query to obtain an object with the text/uri-list media type (see the
IANA registration of this type, which also appears in RFC 2483 [10]).
The query is performed verbatim, with no additional information (such
as URI parameters) added. The server adds the locations contained in
this object to the location set.
CPL servers MAY refuse to allow URI-based sources for location
queries for some or all URI schemes. In this case, they SHOULD reject
the script at script upload time.
There has been discussion of having CPL servers add URI
parameters to the location request, so that (for instance)
CGI scripts could be used to resolve them. However, the
consensus was that this should be a CPL extension, not a
part of the base specification.
Non-URL sources indicate a source not specified by a URL which the
server can query for addresses to add to the location set. The only
non-URL source currently defined is "registration", which specifies
all the locations currently registered with the server.
The "lookup" node also has two optional parameters. The "timeout"
parameter specifies the time, as a positive integer number of
seconds, the script is willing to wait for the lookup to be
performed. If this is not specified, its default value is 30. The
"clear" parameter specifies whether the location set should be
cleared before the new locations are added.
Lookup has three outputs: "success", "notfound", and "failure".
Notfound is taken if the lookup process succeeded but did not find
any locations; failure is taken if the lookup failed for some reason,
including that specified timeout was exceeded. If a given output is
not present, script execution terminates and the default behavior is
performed.
5.2.1 Usage of "lookup" with SIP
For SIP, the "registration" lookup source corresponds to the
locations registered with the server using "REGISTER" messages.
5.3 Location Removal
A CPL script can also remove locations from the location set, through
the use of the "remove-location" node. The syntax of this node is
defined in Figure 11.
The meaning of this node is dependent on the underlying signalling
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protocol.
Node: "remove-location"
Outputs: None (Next node follows directly)
Next node: Any node
Parameters: "location" Location to remove
Figure 11: Syntax of the "remove-location" node
A "remove-location" node removes locations from the location set. It
is primarily useful following a "lookup" node. An example of this is
given in Section 12.8.
The "remove-location" node has one optional parameter. The parameter
"location" gives the URI of a location to be removed from the set, in
a signalling-protocol-dependent manner. If this parameter is not
given, all locations are removed from the set.
The "remove-location" node has no explicit output tags. In the XML
syntax, the XML "remove-location" tag directly encloses the next
node's tag.
5.3.1 Usage of "remove-location" with SIP
The location specified in the "location" parameter of the "remove-
location" node is matched against the location set using the standard
rules for SIP URI matching (as are used, e.g., to match Contact
addresses when refreshing registrations).
6 Signalling Operations
Signalling operation nodes cause signalling events in the underlying
signalling protocol. Three signalling operations are defined:
"proxy," "redirect," and "reject."
6.1 Proxy
Proxy causes the triggering call to be forwarded on to the currently
specified set of locations. The syntax of the proxy node is given in
Figure 12.
The specific signalling events invoked by the "proxy" node are
signalling-protocol-dependent, though the general concept should
apply to any signalling protocol.
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Node: "proxy"
Outputs: "busy" Next node if call attempt returned "busy"
"noanswer" Next node if call attempt was not
answered before timeout
"redirection" Next node if call attempt was redirected
"failure" Next node if call attempt failed
"default" Default next node for unspecified outputs
Parameters: "timeout" Time to try before giving up on the
call attempt
"recurse" Whether to recursively look up
redirections
"ordering" What order to try the location set in.
Output: "busy"
Parameters: none
Output: "noanswer"
Parameters: none
Output: "redirection"
Parameters: none
Output: "failure"
Parameters: none
Output: "default"
Parameters: none
Figure 12: Syntax of the "proxy" node
After a proxy operation has completed, the CPL server chooses the
"best" response to the call attempt, as defined by the signalling
protocol or the server's administrative configuration rules.
If the call attempt was successful, CPL execution terminates and the
server proceeds to its default behavior (normally, to allow the call
to be set up). Otherwise, the next node corresponding to one of the
"proxy" node's outputs is taken. The "busy" output is followed if the
call was busy; "noanswer" is followed if the call was not answered
before the "timeout" parameter expired; "redirection" is followed if
the call was redirected; and "failure" is followed if the call setup
failed for any other reason.
If one of the conditions above is true, but the corresponding output
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was not specified, the "default" output of the "proxy" node is
followed instead. If there is also no "default" node specified, CPL
execution terminates and the server returns to its default behavior
(normally, to forward the best response upstream to the originator).
Note: CPL extensions to allow in-call or end-of-call
operations will require an additional output, such as
"success", to be added.
If no locations were present in the set, or if the only locations in
the set were locations to which the server cannot proxy a call (for
example, "http" URLs), the "failure" output is taken.
Proxy has three optional parameters. The "timeout" parameter
specifies the time, as a positive integer number of seconds, to wait
for the call to be completed or rejected; after this time has
elapsed, the call attempt is terminated and the "noanswer" branch is
taken. If this parameter is not specified, the default value is 20
seconds if the "proxy" node has a "noanswer" or "default" output
specified; otherwise the server SHOULD allow the call to ring for a
reasonably long period of time (to the maximum extent that server
policy allows).
The second optional parameter is "recurse", which can take two
values, "yes" or "no". This specifies whether the server should
automatically attempt to place further call attempts to telephony
addresses in redirection responses that were returned from the
initial server. Note that if the value of "recurse" is "yes", the
"redirection" output to the script is never taken. In this case this
output SHOULD NOT be present. The default value of this parameter is
"yes".
The third optional parameter is "ordering". This can have three
possible values: "parallel", "sequential", and "first-only". This
parameter specifies in what order the locations of the location set
should be tried. Parallel asks that they all be tried simultaneously;
sequential asks that the one with the highest priority be tried
first, the one with the next-highest priority second, and so forth,
until one succeeds or the set is exhausted. First-only instructs the
server to try only the highest-priority address in the set, and then
follow one of the outputs. The priority of locations in a set is
determined by server policy, though CPL servers SHOULD honor the
"priority" parameter of the "location" tag. The default value of this
parameter is "parallel".
Once a proxy operation completes, if control is passed on to other
nodes, all locations which have been used are cleared from the
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location set. That is, the location set is emptied of proxyable
locations if the "ordering" was "parallel" or "sequential"; the
highest-priority item in the set is removed from the set if
"ordering" was "first-only". (In all cases, non-proxyable locations
such as "http" URIs remain.) In the case of a "redirection" output,
the new addresses to which the call was redirected are then added to
the location set.
6.1.1 Usage of "proxy" with SIP
For SIP, the best response to a "proxy" node is determined by the
algorithm of the SIP specification. The node's outputs correspond to
the following events:
"busy" A 486 or 600 response was the best response received to
the call request.
"redirection" A 3xx response was the best response received to
the call request.
"failure" Any other 4xx, 5xx, or 6xx response was the best
response received to the call request.
"no-answer" No final response was received to the call request
before the timeout expired.
SIP servers SHOULD honor the "q" parameter of SIP registrations when
determining location priority.
6.2 Redirect
Redirect causes the server to direct the calling party to attempt to
place its call to the currently specified set of locations. The
syntax of this node is specified in Figure 13.
The specific behavior the redirect node invokes is dependent on the
underlying signalling protocol involved, though its semantics are
generally applicable.
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Node: "redirect"
Outputs: None (No node may follow)
Next node: None
Parameters: "permanent" Whether the redirection should be
considered permanent
Figure 13: Syntax of the "redirect" node
Redirect immediately terminates execution of the CPL script, so this
node has no outputs and no next node. It has one parameter,
"permanent", which specifies whether the result returned should
indicate that this is a permanent redirection. The value of this
parameter is either "yes" or "no" and its default value is "no."
6.2.1 Usage of "redirect" with SIP
The SIP server SHOULD send a 3xx class response to a call request
upon executing a "redirect" tag. If "permanent" was "yes", the server
SHOULD send the response "301" (Moved permanently); otherwise it
SHOULD send "302" (Moved temporarily).
6.3 Reject
Reject nodes cause the server to reject the call attempt. Their
syntax is given in Figure 14. The specific behavior they invoke is
dependent on the underlying signalling protocol involved, though
their semantics are generally applicable.
Node: "reject"
Outputs: None (No node may follow)
Next node: None
Parameters: "status" Status code to return
"reason" Reason phrase to return
Figure 14: Syntax of the "reject" node
A reject node immediately terminates the execution of a CPL script,
so this node has no outputs and no next node.
This node has two arguments: "status" and "reason". The "status"
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argument is required, and can take one of the values "busy",
"notfound", "reject", and "error", or a signalling-protocol-defined
status.
The "reason" argument optionally allows the script to specify a
reason for the rejection.
6.3.1 Usage of "reject" with SIP
Servers which implement SIP SHOULD also allow the "status" field to
be a numeric argument corresponding to a SIP status in the 4xx, 5xx,
or 6xx range.
They SHOULD send the "reason" parameter in the SIP reason phrase.
A suggested mapping of the named statuses is as follows. Servers MAY
use a different mapping, though similar semantics SHOULD be
preserved.
"busy": 486 Busy Here
"notfound": 404 Not Found
"reject": 603 Decline
"error": 500 Internal Server Error
7 Non-signalling Operations
In addition to the signalling operations, CPL defines several
operations which do not affect and are not dependent on the telephony
signalling protocol.
7.1 Mail
The mail node causes the server to notify a user of the status of the
CPL script through electronic mail. Its syntax is given in Figure 15.
Node: "mail"
Outputs: None (Next node follows directly)
Next node: Any node
Parameters: "url" Mailto url to which the mail should be sent
Figure 15: Syntax of the "mail" node
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The "mail" node takes one argument: a "mailto" URL giving the
address, and any additional desired parameters, of the mail to be
sent. The server sends the message containing the content to the
given url; it SHOULD also include other status information about the
original call request and the CPL script at the time of the
notification.
Using a full "mailto" URL rather than just an e-mail
address allows additional e-mail headers to be specified,
such as <mail
url="mailto:jones@example.com?subject=Lookup%20failed" />.
A mail node has only one possible result, since failure of e-mail
delivery cannot reliably be known in real-time. Therefore, its XML
representation does not have output tags: the <mail> tag directly
contains another node tag.
Note that the syntax of XML requires that ampersand characters, "&",
which are used as parameter separators in "mailto" URLs, be quoted as
"&amp;" inside parameter values (see Section C.12 of the XML
specification [2]).
7.1.1 Suggested Content of Mailed Information
This section presents suggested guidelines for the mail sent as a
result of the "mail" node, for requests triggered by SIP. The message
mailed (triggered by any protocol) SHOULD contain all this
information, but servers MAY elect to use a different format.
1. If the "mailto" URI did not specify a subject header, the
subject of the e-mail is "[CPL]" followed by the subject
header of the SIP request. If the URI specified a subject
header, it is used instead.
2. The "From" field of the e-mail is set to a CPL server
configured address, overriding any "From" field in the
"mailto" URI.
3. Any "Reply-To" header in the URI is honored. If none is
given, then an e-mail-ized version of the origin field of
the request is used, if possible (e.g., a SIP "From" header
with a sip: URI would be converted to an e-mail address by
stripping the URI scheme).
4. If the "mailto" URI specifies a body, it is used. If none
was specified, the body SHOULD contain at least the
identity of the caller (both the caller's display name and
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address), the date and time of day, the call subject, and
if available, the call priority.
The server SHOULD honor the user's requested languages, and send the
mail notification using an appropriate language and character set.
7.2 Log
The Log node causes the server to log information about the call to
non-volatile storage. Its syntax is specified in Figure 16.
Node: "log"
Outputs: None (Next node follows directly)
Next node: Any node
Parameters: "name" Name of the log file to use
"comment" Comment to be placed in log file
Figure 16: Syntax of the "log" node
Log takes two arguments, both optional: "name", which specifies the
name of the log, and "comment", which gives a comment about the
information being logged. Servers SHOULD also include other
information in the log, such as the time of the logged event,
information that triggered the call to be logged, and so forth. Logs
are specific to the owner of the script which logged the event. If
the "name" parameter is not given, the event is logged to a standard,
server-defined log file for the script owner. This specification does
not define how users may retrieve their logs from the server.
The name of a log is a logical name only, and does not necessarily
correspond to any physical file on the server. The interpretation of
the log file name is server defined, as is a mechanism to access
these logs. The CPL server SHOULD NOT directly map log names
uninterpreted onto local file names, for security reasons, lest a
security-critical file be overwritten.
A correctly operating CPL server SHOULD NOT ever allow the "log"
event to fail. As such, log nodes can have only one possible result,
and their XML representation does not have explicit output tags. A
CPL <log> tag directly contains another node tag.
8 Subactions
XML syntax defines a tree. To allow more general call flow diagrams,
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and to allow script re-use and modularity, we define subactions.
Two tags are defined for subactions: subaction definitions and
subaction references. Their syntax is given in Figure 17.
Tag: "subaction"
Subtags: Any node
Parameters: "id" Name of this subaction
Pseudo-node: "sub"
Outputs: None in XML tree
Parameters: "ref" Name of subaction to execute
Figure 17: Syntax of subactions and "sub" pseudo-nodes
Subactions are defined through "subaction" tags. These tags are
placed in CPL after any ancillary information (see Section 9) but
before any top-level tags. They take one argument: "id", a token
indicating a script-chosen name for the subaction. The "id" value for
every "subaction" tag in a script MUST be unique within that script.
Subactions are called from "sub" tags. The "sub" tag is a "pseudo-
node": it can be used anyplace in a CPL action that a true node could
be used. It takes one parameter, "ref", the name of the subaction to
be called. The "sub" tag contains no outputs of its own; control
instead passes to the subaction.
References to subactions MUST refer to subactions defined before the
current action. A "sub" tag MUST NOT refer to the action which it
appears in, or to any action defined later in the CPL script. Top-
level actions cannot be called from "sub" tags, or through any other
means. Script servers MUST verify at the time the script is submitted
that no "sub" node refers to any subaction which is not its proper
predecessor.
Allowing only back-references of subs forbids any sort of
recursion. Recursion would introduce the possibility of
non-terminating or non-decidable CPL scripts, a possibility
our requirements specifically excluded.
Every sub MUST refer to a subaction ID defined within the same CPL
script. No external links are permitted.
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Subaction IDs are case sensitive.
If any subsequent version or extension defines external
linkages, it should probably use a different tag, perhaps
XLink [21]. Ensuring termination in the presence of
external links is a difficult problem.
9 Ancillary Information
No ancillary information is defined in the base CPL specification. If
ancillary information, not part of any operation, is found to be
necessary for a CPL extension, it SHOULD be placed within this tag.
The (trivial) definition of the ancillary information tag is given in
Figure 18.
It may be useful to include timezone definitions inside CPL
scripts directly, rather than referencing them externally
with "tzid" and "tzurl" parameters. If it is, an extension
could be defined to include them here.
Tag: "ancillary"
Parameters: None
Subtags: None
Figure 18: Syntax of the "ancillary" tag
10 Default Behavior
When a CPL node reaches an unspecified output, either because the
output tag is not present, or because the tag is present but does not
contain a node, the CPL server's behavior is dependent on the current
state of script execution. This section gives the operations that
should be taken in each case.
no location modifications or signalling operations performed,
location set empty: Look up the user's location through
whatever mechanism the server would use if no CPL script
were in effect. Proxy, redirect, or send a rejection
message, using whatever policy the server would use in the
absence of a CPL script.
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no location modifications or signalling operations performed,
location set non-empty: (This can only happen for outgoing
calls.) Proxy the call to the addresses in the location
set.
location modifications performed, no signalling operations:
Proxy or redirect the call, whichever is the server's
standard policy, to the addresses in the current location
set. If the location set is empty, return "notfound"
rejection.
noanswer output of proxy, no timeout given: (This is a special
case.) If the "noanswer" output of a proxy node is
unspecified, and no timeout parameter was given to the
proxy node, the call should be allowed to ring for the
maximum length of time allowed by the server (or the
request, if the request specified a timeout).
proxy operation previously taken: Return whatever the "best"
response is of all accumulated responses to the call to
this point, according to the rules of the underlying
signalling protocol.
11 CPL Extensions
Servers MAY support additional CPL features beyond those listed in
this document. Some of the extensions which have been suggested are a
means of querying how a call has been authenticated; richer control
over H.323 addressing; end-system or administrator-specific features;
regular-expression matching for strings and addresses; and mid-call
or end-of-call controls.
CPL extensions are indicated by XML namespaces [11]. Every extension
MUST have an appropriate XML namespace assigned to it. The XML
namespace of the extension MUST be different from the XML namespace
defined in Section 14. The extension MUST NOT change the syntax or
semantics of the CPL schema defined in this document. All XML tags
and attributes that are part of the extension MUST be appropriately
qualified so as to place them within that namespace.
Tags or attributes in a CPL script which are in the global namespace
(i.e., not associated with any namespace) are equivalent to tags and
attributes in the CPL namespace "urn:ietf:params:xml:ns:cpl".
A CPL script SHOULD NOT specify any namespaces it does not use. For
compatibility with non-namespace-aware parsers, a CPL script MAY omit
the base CPL namespace for a script which does not use any
extensions.
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A CPL server MUST reject any script which contains a reference to a
namespace which it does not understand. It MUST reject any script
which contains an extension tag or attribute which is not qualified
to be in an appropriate namespace.
A syntax such as
<extension-switch>
<extension has="http://www.example.com/foo">
[extended things]
</extension>
<otherwise>
[non-extended things]
</otherwise>
</extension-switch>
was suggested as an alternate way of handling extensions.
This would allow scripts to be uploaded to a server without
requiring a script author to somehow determine which
extensions a server supports. However, experience
developing other languages, notably Sieve [22], was that
this added excessive complexity to languages. The
"extension-switch" tag could, of course, itself be defined
in a CPL extension.
In the XML schema of CPL, we introduce three abstract elements,
namely `toplevelaction', `switch', and `action', which accordingly
have the abstract type `TopLevelActionType', `SwitchType', and
`ActionType'. Any top-level action in a CPL extension MUST be defined
as the substitutionGroup of the abstract `toplevelaction' element,
and has the type extended from the `TopLevelActionType'. Any switch
in a CPL extension MUST be defined as the substitutionGroup of the
abstract `switch' element, and has the type extended from the
`SwitchType'. Any action in a CPL extension MUST be defined as the
substitutionGroup of the abstract `action' element, and has the type
extended from the `ActionType'.
12 Examples12.1 Example: Call Redirect Unconditional
The script in Figure 19 is a simple script which redirects all calls
to a single fixed location.
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<?xml version="1.0" encoding="UTF-8"?>
<cpl xmlns="urn:ietf:params:xml:ns:cpl"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:ietf:params:xml:ns:cpl cpl.xsd ">
<incoming>
<location url="sip:smith@phone.example.com">
<redirect/>
</location>
</incoming>
</cpl>
Figure 19: Example Script: Call Redirect Unconditional
12.2 Example: Call Forward Busy/No Answer
The script in Figure 20 illustrates some more complex behavior. We
see an initial proxy attempt to one address, with further operations
if that fails. We also see how several outputs take the same action
subtree, through the use of subactions.
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<?xml version="1.0" encoding="UTF-8"?>
<cpl xmlns="urn:ietf:params:xml:ns:cpl"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:ietf:params:xml:ns:cpl cpl.xsd ">
<incoming>
<address-switch field="origin" subfield="user"
xmlns:re="http://www.example.com/regex">
<address re:regex="(.*.smith|.*.jones)">
<reject status="reject"
reason="I don't want to talk to Smiths or Joneses"/>
</address>
</address-switch>
</incoming>
</cpl>
Figure 29: Example Script: Hypothetical Regular-Expression Extension
12.11 Example: A Complex Example
Finally, Figure 30 is a complex example which shows the sort of
sophisticated behavior which can be achieved by combining CPL nodes.
In this case, the user attempts to have his calls reach his desk; if
he does not answer within a small amount of time, calls from his boss
are forwarded to his mobile phone, and all other calls are directed
to voicemail. If the call setup failed, no operation is specified,
so the server's default behavior is performed.
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which scripts are transmitted from a client to a server MUST be
strongly authenticated. Such a method is not specified in this
document.
Script servers SHOULD allow server administrators to control the
details of what CPL operations are permitted.
14 IANA Considerations
This document registers a new MIME type, application/cpl+xml, and a
new URN per RFC 2141 [12], RFC 2648 [13], and RFC 3688 [14].
The XML namespace urn:ietf:params:xml:ns:cpl will only refer to the
version of CPL in this document and will not change. Any CPL
enhancements MUST be made by extensions and MUST have different
namespaces.
14.1 URN Sub-Namespace Registration for urn:ietf:params:xml:ns:cpl
URI: urn:ietf:params:xml:ns:cpl
Registrant Contact: Jonathan Lennox <lennox@cs.columbia.edu>
Xiaotao Wu <xiaotaow@cs.columbia.edu>
Henning Schulzrinne <hgs@cs.columbia.edu>
XML:
BEGIN
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type"
content="text/html;charset=iso-8859-1"/>
<title>Call Processing Language Namespace</title>
</head>
<body>
<h1>Namespace for Call Processing Language</h1>
<h2>urn:ietf:params:xml:ns:cpl</h2>
<p><a href="[[[URL of published RFC]]]">RFCXXXX</a>.</p>
</body>
</html>
END
[Note to RFC Editor: please replace "[[[URL of published
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RFC]]]" above with the official URL of this RFC at rfc-
editor.org, and "XXXX" above with the number of this RFC.]
14.2 Schema registration
This specification registers XML Schema for CPL, as per the
guidelines in [14].
URI: urn:ietf:params:xml:schema:cpl
Registrant contact:
Jonathan Lennox <lennox@cs.columbia.edu>
Xiaotao Wu <xiaotaow@cs.columbia.edu>
Henning Schulzrinne <hgs@cs.columbia.edu>
XML: The XML can be found in Section C.
14.3 MIME Registration
As an XML type, CPL's MIME registration conforms with "XML Media
Types," RFC 3023 [15].
MIME media type name: application
MIME subtype name: cpl+xml
Mandatory parameters: none
Optional parameters: charset
As for application/xml in RFC 3023.
Encoding considerations: As for application/xml in RFC 3023.
Security considerations: See Section 13, and Section 10 of RFC3023.
Interoperability considerations: Different CPL servers may use
incompatible address types. However, all potential
interoperability issues should be resolvable at the time a
script is uploaded; there should be no interoperability
issues which cannot be detected until runtime.
Published specification: This document.
Applications which use this media type: SIP proxy servers and
other telephony servers, and client software to control
their behavior.
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Additional information:
Magic number: None
File extension: .cpl or .xml
Macintosh file type code: "TEXT"
Person and e-mail address for further information:
Jonathan Lennox <lennox@cs.columbia.edu>
Xiaotao Wu <xiaotaow@cs.columbia.edu>
Henning Schulzrinne <hgs@cs.columbia.edu>
Intended usage: COMMON
Author/Change Controller: The IETF.
15 Acknowledgments
This document was reviewed and commented upon by IETF IP Telephony
Working Group. We specifically acknowledge the following people for
their help:
The outgoing call screening script was written by Kenny Hom.
Paul E. Jones contributed greatly to the mappings of H.323 addresses.
The text of the time-switch section was taken (lightly modified) from
RFC 2445 [8], by Frank Dawson and Derik Stenerson.
We drew a good deal of inspiration, notably the language's lack of
Turing-completeness and the syntax of string matching, from the
specification of Sieve [22], a language for user filtering of
electronic mail messages.
Thomas F. La Porta and Jonathan Rosenberg had many useful
discussions, contributions, and suggestions.
Richard Gumpertz performed a very useful last-minute technical and
editorial review of the specification.
A An Algorithm for Resolving Time Switches
The following algorithm determines whether a given instant falls
within a repetition of a "time-switch" recurrence. If the pre-
processing described in Section 4.4.1 has been done, it operates in
constant time. Open-source Java code implementing this algorithm is
available at http://www.cs.columbia.edu/~lennox/Cal-Code/ on the
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world wide web.
This algorithm is believed to be correct, but this section is non-
normative. Section 4.4, and RFC 2445 [8], are the definitive
definitions of recurrences.
1. Compute the time of the call, in the timezone of the time
switch.
2. If the call time is earlier than "dtstart", fail NOMATCH.
3. If the call time is less than "duration" after dtstart,
succeed MATCH.
4. Determine the smallest unit specified in a "byxxx" rule or
by the "freq." Call this the Minimum Unit. Determine the
previous instant (before or equal to the call time) when
all the time units smaller than the minimum unit are the
same as those of "dtstart." If the minimum unit is a
second, this time is the same as the instant. If the
minimum unit is a minute or an hour, the minutes or the
minutes and hours, respectively, must be the same as
"dtstart". For all other minimum units, the time-of-day
must be the same as "dtstart." If the minimum unit is a
week, the day-of-the-week must be the same as "dtstart." If
the minimum unit is a month, the day-of-the-month must be
the same as "dtstart." If the minimum unit is a year, the
month and day-of-month must both be the same as "dtstart."
(Note that this means it may be necessary to roll back more
than one minimum unit -- if the minimum unit is a month,
then some months do not have a 31st (or 30th or 29th) day;
if the minimum unit is a year, then some years do not have
a February 29th. In the Gregorian calendar, it is never
necessary to roll back more than two months if the minimum
unit is a month, or eight years if the minimum unit is a
year. Between 1904 and 2096, it is never necessary to roll
back more than four years -- the eight-year rollback can
only occur when the Gregorian calendar "skips" a leap year.
Call this instant the Candidate Start Time.
5. If the time between the candidate start time and the call
time is more than the duration, fail NOMATCH.
6. If the candidate start time is later than the "until"
parameter of the recurrence (or the virtual "until"
computed off-line from "count"), fail NOMATCH.
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7. Call the unit of the "freq" parameter of the recurrence the
Frequency Unit. Determine the frequency unit enclosing the
Candidate Start Time, and that enclosing "dtstart".
Calculate the number of frequency units that have passed
between these two times. If this is not a multiple of the
"interval" parameter, fail NOMATCH.
8. For every "byxxx" rule, confirm that the candidate start
time matches one of the options specified by that "byxxx"
rule. If so, succeed MATCH.
9. Calculate a previous candidate start time. Repeat until the
difference between the candidate start time and the call
time is more than the duration. If no candidate start time
has been validated, fail NOMATCH.
B Suggested Usage of CPL with H.323
This appendix gives a suggested usage of CPL with H.323 [16]. Study
Group 16 of the ITU, which developed H.323, is proposing to work on
official CPL mappings for that protocol. This section is therefore
not normative.
B.1 Usage of "address-switch" with H.323
Address switches are specified in Section 4.1. This section specifies
the mapping between H.323 messages and the fields and subfields of
address-switches
For H.323, the "origin" address corresponds to the alias addresses in
the "sourceAddress" field of the "Setup-UUIE" user-user information
element, and to the Q.931 [23] information element "Calling party
number." If both fields are present, or if multiple aliases addresses
for "sourceAddress" are present, which one has priority is a matter
of local server policy; the server SHOULD use the same resolution as
it would use for routing decisions in this case. Similarly, the
"destination" address corresponds to the alias addresses of the
"destinationAddress" field, and to the Q.931 information element
"Called party number."
The "original-destination" address corresponds to the "Redirecting
number" Q.931 information element, if it is present; otherwise it is
the same as the "destination" address.
The mapping of H.323 addresses into subfields depends on the type of
the alias address. An additional subfield type, "alias-type", is
defined for H.323 servers, corresponding to the type of the address.
Possible values are "dialedDigits", "h323-ID", "url-ID",
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"transportID", "email-ID", "partyNumber", "mobileUIM", and "Q.931IE".
If future versions of the H.323 specification define additional types
of alias addresses, those names MAY also be used.
In versions of H.323 prior to version 4, "dialedDigits" was known as
"e164". The two names SHOULD be treated as synonyms.
The value of the "address-type" subfield for H.323 messages is "h323"
unless the alias type is "url-ID" and the URL scheme is something
other than h323; in this case the address-type is the URL scheme, as
specified in Section 4.1.1 for SIP.
An H.323-aware CPL server SHOULD map the address subfields from the
primary alias used for routing. It MAY also map subfields from other
aliases, if subfields in the primary address are not present.
The following mappings are used for H.323 alias types:
dialedDigits, partyNumber, mobileUIM, and Q.931IE: the "tel" and
"user" subfields are the string of digits, as is the
"entire-address" form. The "host" and "port" subfields are
not present.
url-ID: the same mappings are used as for SIP, in Section 4.1.1.
h323-ID: the "user" field is the string of characters, as is the
"entire-address" form. All other subfields are not present.
email-ID: the "user" and "host" subfields are set to the
corresponding parts of the e-mail address. The "port" and
"tel" subfields are not present. The "entire-address" form
corresponds to the entire e-mail address.
transportID: if the TransportAddress is of type "ipAddress,"
"ipSourceRoute," or "ip6Address," the "host" subfield is
set to the "ip" element of the sequence, translated into
the standard IPv4 or IPv6 textual representation, and the
"port" subfield is set to the "port" element of the
sequence represented in decimal. The "tel" and "user"
fields are not present. The "entire-address" form is not
defined. The representation and mapping of transport
addresses is not defined for non-IP addresses.
H.323 version 4 [16] defines an "h323" URI scheme. This appendix
defines a mapping for these URIs onto the CPL "address-switch"
subfields, as given in Section 4.1. This definition is also
available as RFC 3508 [24], which is an excerpt from the H.323
specification.
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For h323 URIs, the "user", "host", and "port" subfields are set to
the corresponding parts of the H.323 URL. The "tel" subfield is not
present. The "entire-address" form corresponds to the entire URI.
This mapping MAY be used both for h323 URIs in an h323 "url-ID"
address alias, and for h323 URIs in SIP messages.
B.2 Usage of "string-switch" with H.323
For H.323, the "string-switch" node (see Section 4.2) is used as
follows. The field "display" corresponds to the Q.931 information
element of the same name, copied verbatim. The fields "subject",
"organization", and "user-agent" are not used and are never present.
The "display" IE is conventionally used for Caller-ID
purposes, so arguably it should be mapped to the "display"
subfield of an "address-match" with the field "originator".
However, since a) it is a message-level information
element, not an address-level one, and b) the Q.931
specification [23] says only that "[t]he purpose of the
Display information element is to supply display
information that may be displayed by the user," it seems to
be more appropriate to allow it to be matched in a
"string-switch" instead.
B.3 Usage of "language-switch" with H.323
The language-ranges for the "language-switch" switch are obtained
from the H.323 UUIE "language". The switch is not-present if the
initial message did not contain this UUIE.
B.4 Usage of "priority-switch" with H.323
All H.323 messages are considered to have priority "normal" for the
purpose of a priority switch (see Section 4.5).
B.5 Usage of "location" with H.323
Locations in explicit location nodes (Section 5.1) are specified as
URLs. Therefore, all locations added in this manner are interpreted
as being of alias type "url-ID" in H.323.
Specifications of other H.323 address alias types will require a CPL
extension (see Section 11).
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For location lookup nodes (Section 5.2), the "registration" lookup
source corresponds to the locations registered with the server using
"RAS" messages.
B.7 Usage of "remove-location" with H.323
Location removal nodes (Section 5.3) remove addresses with alias type
"url-ID" using verbatim string matching on the URLs. If a "tel" URL
is specified as the location, matching addresses (ignoring visual
separators) with alias types "dialedDigits" ("e164"), "partyNumber",
"mobileUIM", or "Q.931IE" are also removed. No mechanism is provided
to remove other alias types.
C The XML Schema for CPL
This section includes a full XML Schema describing the XML syntax of
CPL. Every script submitted to a CPL server SHOULD comply with this
XML Schema. When parsing scripts comply with the CPL DTD in earlier
drafts, the DOCTYPE lines in the scripts should be ignored. Note that
compliance with this schema is not a sufficient condition for
correctness of a CPL script, as many of the conditions described in
this specification are not expressible in schema syntax. Figure 31
shows the structure of the schema. `incoming' and `outgoing' are
defined as the substitutionGroup of the `toplevelaction'. All the
switches are defined as the substitutionGroup of the `switch'
element. All the actions are defined as the substitutionGroup of the
`action' element.
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o Fixed bugs in the CPL schema for the format of xs:annotations
element.
o Modified CPL schema for backward compatibilities to the CPL
DTD.
D.3 Changes from Draft -06
The changebars in the Postscript and PDF versions of this document
indicate significant changes from this version.
o Added Xiaotao Wu as a co-author.
o Converted CPL DTD to CPL XML Schema.
o Dropped all features dependent on caller preferences and
callee capabilities.
o Added an XML namespace URN urn:ietf:params:xml:ns:cpl and
registration information for it.
o Separated normative and informative references.
o Updated some references; most notably, updated SIP reference
to RFC 3261. Updated text to reflect changes in these
references.
o Allowed servers more flexibility about recognizing SIP
addresses as telephone numbers.
o Restored some text, in the definition of "interval",
accidentally omitted when sub-day recurrences were re-added in
draft -05.
o Clarified the usages of "lookup" and "remove-location" with
SIP, and "remove-location" with H.323.
o Updated address of the IPTel working group's mailing list.
o Improved wording, cleaned up formatting, and corrected typos.
D.4 Changes from Draft -05
o Clarified that switch nodes are allowed to be degenerate --
they can have no outputs, and they can have only an
"otherwise" output.
o Clarified the (non-) usage of the special language-range "*".
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o Clarified that the Candidate Start Time can be equal to the
call time.
o Modified the DTD to require that the "not-present" output
appear only once.
o Added DTD entries for the "time-switch" attributes re-added in
draft -05.
o Updated the reference to ISO 8601 to cite 8601:2000.
o Updated all H.323 references to cite H.323v4.
o Corrected some spelling errors.
D.5 Changes from Draft -04
o Broke out language switches into their own switch node.
o Restored the full iCalendar COS recurrence specification.
Added text describing the consequences of this for
implementors, and expanded somewhat on the recurrence
algorithm.
o Clarified when time zones are resolved.
o Spelled out "iCalendar" rather than abbreviating it "iCal."
o Clarified some points about host and port matching.
o Whole-address matching in SIP uses the standard SIP URL-match
rules.
o Specified that proxy and lookup timeouts are positive integer
number of seconds.
o Specified that "subaction" "id" parameters must be unique.
o Corrected example scripts' namespace and DTD references
indicating older drafts of this document.
o Deleted an unused subaction from the "Call Forward: Redirect
and Default" example script.
o Made empty switches legal in the DTD.
o Made the legal values for the "proxy" "ordering" parameter
explicit in the DTD.
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o Made the "success" output of "lookup" optional in the DTD. It
can trigger a default action, just like anything else.
o Clarified that the time-switch resolution algorithm is non-
normative.
o Updated references to previously-unpublished RFCs, now
published.
o Thanked Richard Gumpertz.
D.6 Changes from Draft -03
o Removed an obsolete reference to a usage in examples which
wasn't actually used anywhere.
o Added forward references to "remove-location", "mail" and
"log", as well as "location", in the XML syntax as examples of
nodes that don't have explicit output tags.
o Made the usage of some terminology more consistent: "output"
vs. "next node"; "action" vs. "operation" vs. "behavior";
"sub-actions" and "subactions"; "other operations" and "non-
call operations" and "non-signalling operations"; "meta-
information" and "ancillary information."
o The "tel" subfield of addresses which come from sip URIs
should have its visual separators stripped.
o The default value of the "priority" value of the "location"
node is 1.0.
o Corrected the media type of a set of URIs to text/uri-list,
and added a reference to it.
o Added some wording clarifying how URI-based lookup queries
work.
o Corrected the syntax of "duration" parameter in the examples.
o Performed some pre-RFC textual cleanups (e.g. removing the
reference to the Internet-Draft URL from the XML namespace
identifier).
o Re-worded text in the description of the Ancillary tag which
implied that information could be placed in that node in the
base CPL specification. Clarified that the tag is for use by
extensions only.
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o Expunged some references to sub-daily recurrences which had
accidentally been left in the text.
o Updated bibliography to refer to the latest versions of the
cited documents.
o Fixed a number of typographical errors.
D.7 Changes from Draft -02
o Reduced time-switches from the full iCal recurrence to an iCal
subset. Added an appendix giving an algorithm to resolve
time-switches.
o Added the extension mechanism.
o Made explicit how each node is dependent on protocol handling.
Separated out protocol-specific information -- for SIP in
subsections of the main text, for H.323 in a non-normative
appendix.
o Clarified some address mapping rules for H.323.
o Corrected the name of the "Redirecting number" in Q.931.
o Clarified that address matching on the "password" subfield is
case-sensitive.
o Added a recommendation that "tzid" labels follow the usage of
the Olson database.
o Added the "priority" parameter to "location" nodes.
o Added the "default" output to the "proxy" node.
o Made the meaning of the "proxy" node's outputs explicit.
o Added suggested content for the e-mail generated by "mail"
nodes.
o Pointed out that "&" must be escaped in XML (this is relevant
for "mailto" URIs).
o Pointed out that log names are logical names, and should not
be interpreted as verbatim filenames.
o Added some examples.
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Internet Draft CPL April XX, 2004
o Clarified some wording.
o Fixed some typographical errors.
D.8 Changes from Draft -01
o Completely re-wrote changes to time switches: they are now
based on iCal rather than on crontab.
o Timezone references are now defined within time switches
rather than in the ancillary section. The ancillary section is
now empty, but still defined for future use. To facilitate
this, an explicit "ancillary" tag was added.
o Added XML document type identifiers (the public identifier and
the namespace), and MIME registration information.
o Clarified that the "not-present" output can appear anywhere in
a switch.
o Re-wrote H.323 address mappings. Added the "alias-type"
subfield for H.323 addresses.
o Added the "language" and "display" string switch fields.
o Clarified why useless "not-present" outputs can appear in time
and priority switches.
o Added the "clear" parameter to "location" and "lookup" nodes.
(It had been in the DTD previously, but not in the text.)
o Weakened support for non-validating scripts from SHOULD to
MAY, to allow the use of validating XML parsers.
o Added "redirection" output of "proxy" nodes.
o Clarified some aspects of how proxy nodes handle the location
set.
o Added "permanent" parameter of "redirect" nodes.
o Add example script for outgoing call screening (from Kenny
Hom)
o Updated example scripts to use the public identifier.
o Add omitted tag to example script for call forward busy/no
answer
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Internet Draft CPL April XX, 2004
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Internet Draft CPL April XX, 2004
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